1. Field of the Invention
The present invention relates to ablation catheters. In particular, the present invention relates to a linear lesion ablation catheter for producing a generally even temperature profile along the length of the catheter.
2. Description of Related Art
Ablation catheters are well recognized and important tools for conveying an electrical stimulus to selected locations within the human body. Ablation catheters have been used for many years for the treatment of certain types of cardiac arrhythmia. For example, ablation catheters have been used to interrupt or modify existing conduction pathways associated with arrhythmias within the heart. Ablation procedures also are used for the treatment of atrial ventricular (AV) nodal re-entrant tachycardia. Accepted treatments of this condition include ablation of the fast or slow AV nodal pathways. Known cardiac ablation procedures focus on the formation of lesions within the chambers of the heart at selected locations which will either prevent the passage of electrical signals associated with atrial premature contractions or prevent the formation of improper electrical pathways within the heart which can result in atrial arrhythmia.
Radio frequency (RF) catheter ablation has become increasingly popular for many symptomatic arrhythmias such as AV nodal re-entrant tachycardia, AV reciprocating tachycardia, idiopathic ventricular tachycardia, and primary atrial tachycardias. Nath, S., et al., "Basic Aspects Of Radio Frequency Catheter Ablation," J Cardiovasc Electrophysiol, Vol. 5, pgs. 863-876, October 1994. RF ablation is also a common technique for treating disorders of the endometrium and other body tissues including the brain.
A typical RF ablation system in its most basic form comprises an RF generator which feeds current to a catheter containing a conductive electrode for contacting targeted tissue. The system is completed by a return path to the RF generator, provided through the patient and a large conductive plate, which is in contact with the patient's back.
The standard RF generator used in catheter ablation produces an unmodulated sine wave alternating current at frequencies of approximately 500 to 1000 kHz. The RF energy is typically delivered into the patient between the electrode of the catheter and the large conductive plate in contact with the patient's back. During the delivery of the RF energy, alternating electrical current traverses from the electrode through the intervening tissue to the back plate. The passage of current through the tissue results in electromagnetic heating. Heating tissue to temperatures above 50.degree. C. is required to cause irreversible myocardial tissue injury. However, heating tissue to temperatures above approximately 100.degree. C. at the electrode/tissue interface can result in boiling of plasma and adherence of denatured plasma proteins to the ablation electrode. The formation of this coagulum on the electrode causes a rapid rise in electrical impedance and a fall in the thermal conductivity, resulting in loss of effective myocardial heating. Nath, S., et al., "Basic Aspects Of Radio Frequency Catheter Ablation," J Cardiovasc Electrophysiol, Vol. 5, pgs. 863-876, October 1994. Moreover, such extreme heating of the tissues can damage healthy tissue surrounding the targeted lesion.
Ablation catheters for burning lines in tissue are known. Examples of ablation catheters capable of forming linear lesions are shown in U.S. Pat. No. 5,720,775 to Larnard; U.S. Pat. No. 5,528,609 to Swanson; U.S. Pat. No. 5,549,661 to Kordis; U.S. Pat. No. 5,545,193 to Fleischmann; and U.S. Pat. No. 5,575,810 to Swanson. In these known linear ablation catheters, the current density of the linear conductive electrode portion is typically relatively stable in the center of the conductive electrode and tends to approach infinity at the ends of the conductive electrode. These areas of high current density lead to "edge effects" which can cause blood coagulation in these regions, which as stated above, causes a rapid rise in electrical impedance and a fall in the thermal conductivity resulting in a loss of effective myocardial heating. "Edge effects" also can cause extreme heating in the edge areas which can cause undesired tissue damage to healthy tissue surrounding the target tissue.